This invention relates to systems for locating mobile transceivers.
In order to locate a target mobile transceiver in the coverage area of a communications network, it is necessary for a location system to be able to detect a signal transmitted from the target mobile transceiver. The location system uses measurements of the signal transmitted by the target mobile transceiver such as the time of arrival (TOA) to locate the mobile transceiver. In a system where there are many mobile transceivers and the location system does not have access to identification codes used within the communications network to identify the mobile transceivers, this problem becomes difficult. The location network may know the call sign or telephone number allocated to the mobile transceiver, and thus be able to call the mobile transceiver. However, the location system does not necessarily know which of the many signals being transmitted within the communications network by multiple unknown mobile transceivers corresponds to the transmissions of the target mobile transceiver. The location system requires some way to identify signals transmitted by the target mobile transceiver and differentiate those signals from all other signals being used by other mobile transceivers in the communications system. This invention provides a solution to this problem.
In one aspect, this invention provides a method of identifying a target mobile transceiver in a communications network that includes multiple unknown mobile transceivers. In this aspect of the method, a location system interrogates the target mobile transceiver multiple times in an interrogation pattern to cause the target mobile transceiver to transmit a signal in response to each interrogation. The location system monitors signals transmitted by mobile transceivers in at least a portion of the network, and identifies the target mobile transceiver as the mobile transceiver whose pattern of response exceeds a matching threshold and most closely matches the interrogation pattern.
Preferably, to avoid computational overload, signals are monitored in a portion of the network, and, upon failure of any monitored signals to exceed the matching threshold, signals transmitted by mobile transmitters are monitored in another portion of the network. In addition, to assist in avoiding false positives, monitoring may be limited to a monitoring interval immediately following the interrogation.
In a further aspect of the invention, upon identifying the target mobile transceiver, information is generated that specifies the location of the target mobile transceiver. The step of interrogating may be initiated after a request from a requester and the location information generated is provided to the requestor.
These and other aspects of the invention are described in the detailed description of the invention and claimed in the claims that follow.